Since all cells in the heart contract in every cardiac cycle, each cell must be capable of different states of contractility. This project will focus on regulation by changes in the contractile proteins themselves especially by means of phosphorylation. Studies will be conducted on bundles of rat ventricular fibers that have been made hyperpermeable by treatment with an agent that chelates divalent cations. These hyperpermeable fibers retain their intracellular and many of their membranes proteins. They offer the possibility of directly probing the contractile proteins with Ca buffers in a preparation that resembles the intact cell more closely than any skinned preparation. Experiments that have been conducted so far indicate that both the Ca sensitivity and the contractility of the contractile proteins can be varied by cyclic nucleotides. Cycic GMP increases and cAMP decreases Ca sensitivity. Epinephrine, cAMP and guanosine triphoshate can each increase contractility. This project will continue the studies of these kinds of regulation in order both to define the control systems more precisely and to examine the proteins directly.